Motor imagery is an active cognitive process that has measurable effects on human motor performance. The latent behavioral effect resulting from motor imagery could be expressed after minimum physical practice. Using motor imagery as a potential efficient intervention to promote early recovery is promising particularly when patients could not start physical rehabilitation in an early stage. Accumulated evidence has demonstrated that imagined and actual movements share similar neural substrates, including the motor cortex. However, the relations between the motor centers/pathways during imagined and executed movements are rarely studied. In the proposed study, we plan to examine specificity of the involvement of the motor system during motor imagery in normal subjects. Based on our previous findings, we hypothesize that motor centers/ pathways for voluntary movements are facilitated at subthreshold levels during motor imagery and that these are specifically linked to actual movement of the same action in an obligatory manner. The specific Aims are: 1) to analyze movement-specific subthreshold activation of motor pathways of the target muscles during motor imagery using a reaction time paradigm; 2) to examine the interactions between motor imagery and motor execution. In Aim 1, we will compare the premotor time (PMT) at rest, during different background imagined movements, and during muscle activation to investigate the movement-specific effect of motor imagery. During reaction time movements, the effect of motor imagery on the PMT could appear as the sum of the excitatory effect due to enhanced corticospinal excitability specifically for the prime mover of the imagined movement and an overall inhibition associated with increased task complexity during motor imagery. The concomitant changes in the corticospinal excitability will be further investigated using appropriately timed transcranial magnetic stimulation (TMS). In Aim 2, we will examine the interactions between movement-specific subthreshold activation during motor imagery and muscle activation during actual movement by comparing the responses to TMS. Due to a centrally programmed pattern of reciprocal inhibition, we expect that the responses to TMS in the preactivated finger flexors will be decreased during imagined finger extension, but may not be further increased during imagined finger flexion. [unreadable] [unreadable] [unreadable] [unreadable]